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IEEE Photonics Conference

Cascaded Quantum Well Structure Emitting Entangled Multiphoton States

Amir Sivan, Meir Orenstein

Published Nov 12, 2023• 0 citations
Publisher

Abstract

Deterministic cascaded quantum well source spontaneously emits inter-entangled triple frequency multiphoton states. The entanglement stems from interference of indistinguishable quantum evolution paths in the cascaded coupled quantum wells. Novel controllable quantum entanglement and quantum error detection are two possible features of the multiphoton source.

DOI
10.1109/ipc57732.2023.10360765

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Deterministic cascaded quantum well source spontaneously emits inter-entangled triple frequency multiphoton states. The entanglement stems from interference of indistinguishable quantum evolution paths in the cascaded coupled quantum wells. Novel controllable quantum entanglement and quantum error detection are two possible features of the multiphoton source.
Entangled multiphoton states are a critical resource for the emerging applications of quantum science. Generation of reliable sources of such quantum states of light poses both theoretical and practical challenges, particularly in the small scale. We propose a deterministic source of entangled multiphoton states based on spontaneous emission from a ladder of cascaded quantum potential wells. The special ingredient of this scheme compared with a simple ensemble of emitters is the coupling between the two-level systems, creating sublevels in the energy ladder. This enables, via an indistinguishable multipath evolution, the creation of highly entangled multiphoton photon-number combination states in three modes, namely, tripartite entanglement in the frequency degree of freedom. Due to the intricate energy ladder of our proposed structure, we can apply control and switch the entanglement between two multiphoton modes by measuring the third. We further discuss an application as a qubit-pair source with an error-detection ancilla and comment on possible implementations.
The lack of translational invariance perpendicular to the plane of a single quantum well causes equal probability for spontaneous emission to the left or right. Combining one emission path from the left and one from the right into a common detector leads to interference fringes for fundamentally indistinguishable paths corresponding to geometries where the same in-plane momentum is transferred to the quantum well. For all other paths, no interference is observed because of the entanglement between the photon and extended Bloch states of the many-body system. In multiple-quantum-well structures the interference can be controlled via the spacing between the wells.
A well controlled, triggered source of entangled photons is desirable for many applications in quantum information processing. The two-photon cascade from a biexciton state, two electrons and holes, confined in a quantum dot can be such a source of polarisation-entangled photons provided the two decay paths from the biexciton carry no “which-path” information. In recent years many techniques have been employed to make the two optical decay paths from the biexciton state indistinguishable. A number of these techniques will be discussed and entangled-photon emission from a single quantum dot with a high fidelity in the expected Bell-state from the cascade will be demonstrated. This source of entangled photons allows optical interferometry beyond the limits imposed by the photon wavelength. Interference fringes of the entangled biphoton state reveals a periodicity half of that obtained with the single photon, and much less than that of the pump laser. High fringe visibility indicates that biphoton interference is less sensitive to decoherence than interference of two sequential single photons. The effect of the exciton fine-structure splitting on our entangled photon source will be shown. Surprisingly the entanglement is found to persist despite relatively large separations between the intermediate energy levels of up to 4µeV. Measurements demonstrate that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first order coherence time of either single photon. We distinguish between the first-order coherence time, and a parameter defined as the cross-coherence time, this is illustrated in the figure.
We describe a method to detect twin-beam multiphoton entanglement based on a beam splitter and weak nonlinearities. For the twin-beam four-photon entanglement, we explore a symmetry detector. It works not only for collecting two-pair entangled states directly from the spontaneous parametric down-conversion process, but also for generating them by cascading these symmetry detectors. Surprisingly, by calculating the iterative coefficient and the success probability we show that with a few iterations the desired two-pair can be obtained from a class of four-photon entangled states. We then generalize the symmetry detector to n-pair emissions and show that it is capable of determining the number of the pairs emitted indistinguishably from the spontaneous parametric down-conversion source, which may contribute to explore multipair entanglement with a large number of photons.
Photon indistinguishability, entanglement, and antibunching are key ingredients in quantum optics and photonics. Decay cascades in quantum emitters offer a simple method to create entangled-photon-pairs with negligible multipair generation probability. However, the degree of indistinguishability of the photons emitted in a cascade is intrinsically limited by the lifetime ratio of the involved transitions. Here we show that, for the biexciton-exciton cascade in a quantum dot, this ratio can be widely tuned by an applied electric field. Hong-Ou-Mandel interference measurements of two subsequently emitted biexciton photons show that their indistinguishability increases with increasing field, following the theoretically predicted behavior. At the same time, the emission line width stays close to the transform-limit, favoring applications relying on the interference among photons emitted by different sources.